Method and device for manufacturing ultrafine fibres from thermoplastic polymers

ABSTRACT

A process and device for manufacturing ultrafine fibers and ultrafine-fibre mats from thermoplastic polymers with mean fibre diameters of 0,2-15 μm, preferably 0,5-10 μm, by a melt blowing technique. The polymer melt (12) flows through at least one bore (15) in a melt blowing nozzle (18). Immediately on emerging from the bore, gas is blown against the extrusion from both sides of the bore exit (15), thus breaking up the melt to form fibers. To this end, the gas is accelerated to supersonic speed in Laval nozzles (25, 26; 31, 32), disposed in mirror symmetry round the bore exits (15), and decelerated to just below the speed of sound in channels (27) with constant cross-section, or a cross-section which decreases in the direction of flow, fitted downstream of the Laval nozzles, and the melt (12) fed into the gas stream emerging from the channels (27).

BACKGROUND OF THE INVENTION

The invention is based on a process for producing microfibres andnon-woven microfibre webs from thermoplastic polymers by themelt-blowing technique in which a polymer melt flows through at leastone orifice in a melt die and is separated into fibers by a gas whichimpinges on the melt from both sides immediately after its exit from theorifices. The invention also relates to a device for carrying out theprocess. The melt-blowing process has been disclosed in numerouspublications (see e.g. U.S. Pat. Nos. 3,755,527, 3,978,185, 4,622,259and 3,341,590), and German Patent No. 2,948,821. According to themelt-blowing technique a stream of polymer melt extrusion issuing from amelt orifice is separated into individual fibers and drawn out whileattenuated by means of an inert gas, in most cases air, which has atemperature higher than or equal to the temperature of the melt and isblown against the melt in the direction of flow. One main object is toincrease the economic efficiency of the process by appropriatelyregulating the melt viscosity. Thus the prior art discloses the use ofpolymers with an extremely low viscosity and correspondingly highextrusion flow rates, since this enables relatively fine fibers to beproduced with a lower degree of energy consumption by reducing thetemperature of the melt and the gas stream. The following parameters areknown to have a crucial effect on the economic efficiency of theprocess:

a) The number of melt orifices (per unit of length) and the throughputof the melt per orifice,

b) the melt temperature and viscosity of the melt,

c) the gas inlet pressure for obtaining a uniform gas stream with highflow rate over the whole length of the die,

d) the temperature of the gas stream, and

e) the mass flow rate of the gas.

According to the prior art the gas temperature is adjusted to a valuehigher than or equal to the temperature of the melt. In all knownprocesses the gas stream issues from the die in direct proximity to themelt orifices and on either side thereof via exit slots arranged in thelongitudinal direction of the die. Complicated hydrodynamic brake meansand air distributing systems have to be provided in the gas inlets toensure a uniform rate of flow over the entire slot length. PCTapplication WO 87/04195 describes appropriate technical means forachieving optimum results.

The use of relatively large gas exit slots (1 mm to 3 mm) has also beendisclosed. One disadvantage of this method is the high quantity of gasrequired since high rates of flow are necessary in particular for theproduction of very fine fibers of an average diameter of <3 μm. The rateof flow at the slot exit is usually 0.5 to 0.7 times the sonic speed ofthe gas (0.5 V_(s) to 0.7 V_(s) ; V_(s) =sonic speed).

SUMMARY OF THE INVENTION

A principle object of the invention is to provide an additional increasein the economic efficiency of the melt-blowing process. In particularthe object is to provide higher economic efficiency in the production offibers of mean fibre diameters of <10 μm, preferably <5 μm. Anadditional object is to considerably increase the melt throughput perorifice and thus the total spinning capacity of the installation whenproducing fibers of mean diameters of between 0.5 μm and 3 μm.

According to the invention the above objects are achieved byaccelerating the rate of flow of the gas to supersonic speed in Lavalnozzles arranged mirror-symmetrically in relation to the melt orificesand reducing the rate of flow of the gas in flow channels arrangeddownstream of the Laval nozzles, and having a constant cross-section ora cross-section tapering in the direction of flow to a rate just belowsonic speed and by directing the polymer melt into the gas streamissuing from the flow channels. "Just below sonic speed" is understoodto be a range between 0.8 V_(s), preferably 0.9 V_(s) and 0.99 V_(s)(0.8 V_(s) <V<0.99 V_(s), preferably 0.9 V_(s) <V<0.99 V_(s)). Whereasin the known melt-blowing process the rate of flow of the gas stream atthe exit to the slot nozzles is considerably lower than sonic speed, thesolution provided by the invention is based on a gas exit speed justbelow sonic speed ("transonic speed") which is obtained in a particularmanner. This solution is effected technically with the aid of Lavalnozzles which are oriented in the direction of the gas stream adjacentto the tip of the melt die and are arranged at a small distance upstreamof the melt orifices. Thus the device for carrying out the process ischaracterised according to the invention in that the gas nozzles aredesigned in the form of Laval nozzles with flow channels arrangeddownstream thereof and having a convergent or constant cross-section,which are arranged in direct proximity to the wedge-shaped die tip andterminate with a sharp edge maximally 3 mm above or below the level ofthe melt orifices.

The Laval nozzles can either have a square or a circular cross-sectionwith an orifice diameter of 0.3 to 2 mm.

Preferably widened sections leading into the flow channel are arrangeddownstream of the Laval nozzles. The inlet cross-section of the flowchannel should be 1 to 2.5 times the sum of the widened cross-sectionsof the Laval nozzles and the length of the flow channels should be 1 to30 times the widened cross-section.

According to a further embodiment a gas smoothing chamber (tranquilizingchamber) is arranged upstream of the Laval nozzles and several linearlyarranged Laval nozzles are assembled together with the corresponding gassmoothing chambers in the form of individual units to form a modular gassupply element.

The gas supply elements, whose width is 25 mm to 500 mm and preferably50 mm to 200 mm are advantageously connected in a gas-tight manner bothto the melt die and to each other. In a further advantageously designedmodification the gas supply elements can be displaced parallel to thewedge-shaped contour of the melt die tip in order to allow theadjustment of the distance between the Laval nozzles and the meltorifices.

Compared to the previously known melt-blowing processes a considerablyhigher space time yield (production rate) is achieved under stable anduniform operating conditions. Of considerable importance is thereduction in the rate of flow of the gas streams which issue from theLaval nozzles at supersonic speed, in the flow channels arrangeddownstream of the Laval nozzles. The flow channels are constructed insuch a manner that one side of each flow channel is formed by the outerwall of the melt die tip. The Laval nozzles and the flow channels can bedisplaced parallel to the outer walls of the wedge-shaped melt die so asto allow the use of either position typically employed in themelt-blowing technique, i.e. either the stick out or the set backposition. The following advantages are obtained by the invention:

1. Due to the presence of the particularly uniform transonic region offlow in the vicinity of the melt orifices the draw rate of the melt fromthe orifices and thus also the yield, is greatly increased withouttrading off between product quality and yield.

2. It has been found that it is possible to considerably increase themelt throughput per hole for fibre thicknesses of less than 5 μm and inparticular less than 3 μm.

3. It has also been found that, compared with the conventional processfor the production of fibers of the same fineness, considerably lowergas throughput quantities are required for identical melt throughputquantities.

4. Static pressures in the gas smoothing chamber of less than 4 bar(abs.), and preferably less than 2.5 bar (abs.) are sufficient forobtaining a transonic region of flow.

5. In the Laval nozzles the gas is distributed over the length of thedie in an absolutely uniform manner, so that additional means forensuring uniformity which are necessarily associated with a loss inpressure can be dispensed with.

6. Compared to the conventional melt-blowing process the specific energyconsumption can be reduced by a factor of 2 for an identical fibrefineness in the range d<=5 μm, preferably <=3 μm.

7. Due to the reduced quantity of gas required the fibers can bedeposited more uniformly and without any secondary entangling on to thefibre-collecting belt, especially in the case of very fine fibers. Alsoflying fibers are avoided in the production of very fine fibers (<2 μm)and low web densities.

8. Due to the increased rate of attenuation in the transonic region offlow the gas temperature can be considerably decreased in comparisonwith the conventional process for producing identical fibre thicknesses.As a result of the reduced quantity of gas there is also lesscompression of the web material as it is deposited on to thefibre-collecting belt; i.e. a web with reduced density is producedwithout adhesion of the fibers.

9. Due to the absolutely uniform distribution of gas over the width ofthe die disadvantageous edge zone effects can be avoided.

10. The process has proven particularly effective for the production offibre webs with fibre finenesses of less than 3 μm, and in particularless than 2 μm.

11. The fibre webs produced by the process have excellent filtrationproperties as a result of their reduced density and homogeneousstructure. Compared to prior art the following superior results of suchfilter webs are predominant:

a. higher particle filtration efficiency at a reduced flow resistance

b. a higher dust collecting capacity,

c. higher electrostatic charge accumulating capacity, for example if anelectric corona discharge technique is used for charging the web.

In the following, working examples of the invention are explained inmore detail with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically the layout of a complete melt-blowinginstallation

FIG. 2 shows an embodiment of the melt-blowing die according to theinvention, including the gas supply elements (lateral view)

FIG. 3 shows a magnified section of the melt-blowing die with circularLaval nozzles

FIG. 4 shows a magnified section of the melt-blowing die withslot-shaped Laval nozzles

FIG. 5 is a perspective view of a melt-blowing die with air supplyelements in modular form

FIG. 6 shows the specific energy consumption of the process as afunction of the average fibre diameter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Sheet fibre products, in particular fleece materials or fibre webs,manufactured by the melt-blowing process are of great economicimportance in present-day technology. They are used for manyapplications and in particular in cases where very fine fibres arerequired in conjunction with high surface coverage. Virtually allmelt-extrudable thermoplastic polymers can be used as startingmaterials. Possible applications are for example: Filtration media,hygienic filters, medical applications, protective clothing, absorbentmedia, battery separating media, insulating clothing etc. Materialscombined with other textiles or non-woven webs are also known. It isthus highly important to improve the economic efficiency of themelt-blowing process. An increase in the melt throughput rate and/or areduction in the specific air consumption is a necessary requirement forachieving an improvement in economic efficiency. It goes without sayingthat product quality must not in any way suffer as a result of suchimprovements; i.e. product quality must at least remain constant. Forthe production of filtration media with a high degree of filtrationefficiency and low flow resistance microfibre media are required havinga lower density at identical or higher fibre fineness than thatdisclosed in the prior art. It is also advantageous to be able toproduce the sheet fibre products at lower gas and melt temperatures thanthose employed in the prior art. This permits a reduction in thetendency of the fibres to adhere to each other on being deposited on thefibre collecting belt and simultaneously decreases the tendency oftemperature-sensitive polymers to undergo thermal decomposition duringthe extrusion and spinning process and at the same time increases thelifetime of the spinning nozzles. In order to obtain uniform andhomogeneous product quality over the entire width of the web anabsolutely uniform and constant distribution of air with regard to spaceand time is required.

The production of a fibre web by the melt-blowing process is firstdescribed generally (i.e. according to the prior art) with reference toFIG. 1. The extruder 1 driven by a motor 2 is fed with a polymer viafunnel 3. The polymer melt is delivered to the melt-blowing die 6 viamelt filter 5 by means of a spinning pump 4. The extruder, the spinningpump, the melt filter, the die and the transition zones are heated inorder to obtain the required temperature and viscosity of the melt. Themelt-blowing nozzle 6 has inlets for the fibre-forming gas 7 which issupplied by means of a compressor and is heated to the requiredtemperature by means of a heat exchanger (not depicted) before it entersthe melt-blowing die 6. The melt-blowing die 6 has at least one linearrow of fine orifices from which the melt issues by means of an inletpressure produced by the spinning pump 4 and is attenuated by means ofgas 7 to form microfibres which are deposited on a mechanically drivenfibre-collecting belt 9 to form the finished web 10. A portion of thegas stream is removed by means of a suction box 11 arranged beneath thefibre-collecting belt 9. FIG. 2 shows a cross-section through theembodiment of the melt-blowing die on which the invention is based. Thepolymer melt 12 flows into slot 14 via melt distributor 13 and then intothe orifices 15 from which it issues while being attenuated intomicrofibres by means of a gas 16 (air) supplied from both sides at ahigh rate of flow. The melt distributor 13 is arranged inside a dieblock 17, below which the melt-blowing die 18 is arranged in amelt-tight manner. The die block 17 and the melt-blowing die 18 areheated by means of electric resistance wires 19 arranged within thesurrounding heating jacket 20. The wedge-shaped die tip 21 of themelt-blowing die 18 has an angle of 20° to 100°, preferably 40° to 80°.The melt exit holes 15 are arranged linearly (perpendicularly to thedrawing plane) and have a diameter of 0.1 to 0.6 mm, preferably 0.1 to0.4 mm, and a channel length 2 to 10 times their diameter.

The fibre-forming gas 16 is fed from both sides via openings 22 into gassmoothing chambers 23 arranged inside the gas supply elements 24. Thesmoothing chambers 23 lead into very small, linearly arranged gasopenings 25, which are located in direct proximity to the die tip 21 andoriented in a direction parallel to the wedge-shaped contour of the dietip 21. The gas openings 25 are provided with widened sections 26 andrepresent fluidically (with regard to the flow configuration) widenedLaval nozzles (25, 26). A flow channel 27 is arranged downstream of eachof the widened sections 26 which is defined on the one side by thecontour of the melt die tip 21 and on the other side by the bottomplates 28, the bottom plates 28 terminating with a sharp edge in theregion of the apex of the die tip 21. The gas supply elements 24together with the smoothing chambers 23 and the Laval nozzles 25, 26 arearranged on either side of the melt orifices 15 or die axis 29 andmirror-symmetrically in relation thereto.

The gas supply elements 24 are arranged adjacently to the contour of thewedge-shaped die tip 21 in a gas-tight manner and can be displaced in aparallel direction to the wedge-shaped surfaces. It is thus possible toadjust the distance between the Laval nozzles 25, 26 and the meltorifices 15. Depending on the polymer specifications and the requiredphysical web properties it is therefore possible to displace the outletof the melt orifices 15 in relation to the sharp-edged outlet of theissuing gas jets to the required extent upwards or downwards in thedirection of flow. The bottom plates 28 can also be displacedtransversely to the die axis 29, thus allowing the flow slot 30 or theflow channels 27 to be accurately adjusted. The gas openings 25 of theLaval nozzles have a diameter of 0.3 to 2.0 mm, preferably 0.4 mm to 1mm, and a length 0.3 to 5 times the diameter. The widened section 26beneath the gas openings 25 has a total angle of 5° to 30°, preferably10° to 20°. The widened section 26 is conically shaped, and is eitheraxially symmetrical to the axis of the gas opening 25 or is inclined atan angle in relation thereto (as shown in FIG. 3). The latter embodimenthas the advantage that the Laval nozzles 25, 26 can be arranged indirect proximity to the die tip 21. The cross-section of the flowchannels 27 downstream of the Laval nozzles converges or remainsconstant in the direction of flow. The length of the flow channels 27 is1 to 30, preferably 3 to 20 times the largest diameter of the widenedsections 26 of the Laval nozzles. The main purpose of the flow channelsis to form a homogeneous region of transonic flow in the longitudinaldirection of the flow channels 27.

With the aid of the Laval nozzles 25, 26 and by establishing a pressureratio between the flow channel 27 and the gas smoothing chamber 23 whichcorresponds at least to the critical Laval pressure ratio of 0.53, arate of flow is formed in the Laval orifice 25, as a result of the knownflow parameters, which corresponds to sonic speed at the giventemperature. This parameter applies to all Laval nozzles 25 so thatthroughout the length of the melt-blowing die 18 (perpendicularly to theplane of projection) an absolutely uniform stream of gas issues from theflow slot 30. Inlet pressures in the gas smoothing chambers 23 of 1.9 to5 bar (abs.), preferably 1.9 to 2.5 bar (abs.) are sufficient forobtaining such flow conditions. The widened section 26 of the Lavalnozzles serves to accelerate the flow to supersonic speed and to improvethe cross-sectional homogeneity of the gas stream as it enters flowchannel 27. Due to the parallel or convergent shape in flow channel 27the ultrasonic diffusion effect causes the rate of flow to be reduced totransonic speed with optimum cross-sectional homogeneity in theproximity of the flow slot 30. "Transonic speed" is understood to referto a flow rate of at most 20% (at maximum), and preferably at most 10%below sonic speed. The inlet cross-section of the flow channels 27 is1.0 to 2.5 times the sum of the cross-sections of the widened sections26 of the Laval nozzles and the outlet cross-section is 0.8 to 2.5 timesthis sum. Such conditions provide a high degree of flow stability andhomogeneity in the critical region of the orifices 15.

FIG. 3 shows the assembly of the gas smoothing chamber 23, the Lavalnozzle 25, 26 and the flow channel 27 once again in magnified form. Thewall thickness of the portion of the gas supply element 24 adjacent tothe outer wall of the die tip 21 at the level of the gas openings 25(Laval nozzles) is as small as technically possible. The sharp-edgedoutlet to the flow channel 27 (referred to as the flow slot 30) is flushwith the melt orifice in this embodiment. The gas smoothing chamber 23begins with a relatively large cross-section and tapers continuouslytowards the Laval orifices 25, thus helping to minimise flow resistancein the subsonic region. The distance a, i.e. the length of the flowchannel 27, is in the range from 1 mm to 50 mm, preferably 2.5 mm to 30mm.

FIG. 4 shows an alternative slot-shaped embodiment of the Laval nozzles.Both the Laval orifice and the widened section downstream thereof areslot-shaped in this embodiment. Thus the Laval nozzle consists of Lavalslot 31 and the slot-shaped widened shaft 32 downstream thereof. Theslot-shaped cross-section of the Laval nozzles 31, 32 extends over thewhole width of the die tip (perpendicularly to the drawing plane). Thewidened shaft has a total angle of 5° to 30°, preferably 10° to 15°. Asin the embodiment according to FIG. 2 a flow channel 27 with aconvergent or constant cross-section which terminates with slot 30, isarranged downstream of the widened shaft 32. In all of the embodimentsshown in FIGS. 2 to 5 the fibre-forming gas which produces the fibresand attenuates the melt streams issuing from the melt orifices 15, isformed by gas streams directed on to the melt strands from both sides bymeans of flow channels 27.

FIG. 5 shows a particularly advantageous modular construction in which anumber of air supply elements 33a, 33b, 33c, 33d . . . are arranged nextto or behind one another at the side of the melt-blowing die 18 in theform of an assembly of individual units. Each unit is connected via pipe34a, 34b . . . to a distributor pipe 35 which is supplied with thefibre-forming gas 16. Each gas supply element comprises a gas smoothingchamber 23 which supplies several Laval nozzles 25, 26 with a circularcross-section or one slot-shaped Laval nozzle 31, 32.

The gas supply elements 33a, 33b . . . are sealed at their front ends,so that they represent individually effective units which are juxtaposedto each other in a gas-tight manner. As shown in FIG. 5 and inaccordance with the basic embodiment according to FIG. 2 the gas supplyelements are arranged mirror-symmetrically (to the central plane of themelt-blowing die 18) on either side of the die tip 21.

The embodiment according to FIG. 5 has the following advantagesespecially for the production of fibre webs of large widths:

1. The gas stream in slots 30 is absolutely uniform over the wholewidth, even where dies of large dimensions are employed.

2. Provided the width of the individual units is not too large,misalignment of the Laval openings 25 or the Laval slot 31 during themanufacture of the Laval nozzles can be avoided. Appropriate unit widthsare in the range from 25 to 500 mm, preferably 50 to 200 mm.

3. The unit assembly allows the air supply elements to be connected tothe melt-blowing die 18 in the best possible manner.

4. Webs of different widths can be obtained in a simple manner.

EXAMPLE

Polypropylene produced by Exxon, Type PD 3495 having a melt flow indexof 800 g/10 min was melted according to FIG. 1 and delivered to amelt-blowing die according to FIGS. 2 and 3 having the followingcharacteristic dimensions:

Diameter of the melt orifices 15: 0.3 mm

Channel length: 3.8 mm

Lateral distance of melt orifices 15: 1.25 mm

Apex angle of the melt die tip 21: 60°

Diameter of the Laval orifice 25: 0.6 mm

Length of the Laval orifice 25: 0.3 mm

Widened section 26 of the Laval nozzle: Total angle 15°;

End diameter: 0.7 mm

Lateral distance of the gas openings 25: 0.8 mm

Flow channel 27: initial width 0.8 mm; width at exit (at the level ofthe sharp-edged outlet): 0.7 mm;

Length: 2.3 mm

Unit width: 50 mm

Number of units: 2 on each side.

The sharp-edged outlet of the air exit slot (flow slot) 30 was flushwith the die tip 21. Air was used as the fibre-forming gas; it wascompressed in a screw compressor and heated to the required temperaturein an electric heater arranged downstream of the compressor.

During web formation a portion of the volumetric stream of thefibre-forming gas was removed with the aid of the suction removal means11.

Table 1 shows the results obtained in relation to fibre load, fibrediameter and specific energy consumption, the apparatus was operatedwith the following process parameters:

Static pressure of the air in the gas smoothing chamber 23: 3 bar(abs.),

Temperature of the air: 285° C.,

Melt temperature: 230° C.,

Inlet pressure of the melt upstream of the filter 5 (see FIG. 1): 35bar.

With the above process parameters a sonic speed of about 440 m/s resultsin the Laval nozzles and a rate of flow of about 5% below sonic speed isobtained at the flow slot 30. The distance between the melt-blowingnozzle 21 and the fibre-collecting belt 9 was 0.3 m. Table 2 shows theresults of a further series of tests, in which the static pressure ofthe air in the smoothing chambers 23 was decreased to 2.2 bar (abs.) andthe gas temperature was increased to 294° C. No changes were made in theremaining operating parameters.

In the tables:

m_(F),B is the melt mass throughput per hole,

λ is the air stream load (ratio of the mass flow rate of the fibres tothe mass flow rate of the blowing air)

1/λ is the consumption of blowing air in relation to the quantity offibres produced d_(F) is the mean fibre diameter

E_(L) /m_(F) is the specific net energy consumption necessary forcompressing and heating the blowing air, based on the quantity of fibresand at an inlet temperature of 40° C. of the air into the electric airheater.

                  TABLE 1                                                         ______________________________________                                                λ                                                              . m.sub.F,B                                                                           *10.sup.-3                                                                              1/λ  d.sub.F                                                                            E.sub.L /. m.sub.F                         (g/min) (kg.sub.F /kg.sub.L)                                                                    (kg.sub.L /kg.sub.F)                                                                      (μm)                                                                            (kWh/kg.sub.F)                             ______________________________________                                         0,089   3,37     297         1,25 36,6                                       0,16    6         165,6       1,68 20,4                                       0,23     8,7      114,8       2    14,2                                       0,3     11,3      88,8        2,3  11                                         0,43    16,2      61,7        2,6   7,6                                       0,56    21,1      47,3        2,95  5,8                                       0,87    32,9      30,4        3,25  3,7                                       ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                λ                                                              . m.sub.F,B                                                                           *10.sup.-3                                                                              1/λ  d.sub.F                                                                            E.sub.L /. m.sub.F                         (g/min) (kg.sub.F /kg.sub.L)                                                                    (kg.sub.L /kg.sub.F)                                                                      (μm)                                                                            (kWh/kg.sub.F)                             ______________________________________                                         0,086   4,47     223,8       1,44 25,1                                       0,15     7,9      126,6       1,61 14,2                                       0,23    12        83          1,77 9,3                                        0,3     15,8      63,3        2,11 7,1                                        0,43    22,2      45,1        2,65 5,1                                        0,57    29,6      33,8        3,1  3,8                                        ______________________________________                                    

In both test series it was found that very fine fibres can be obtainedwith very efficient energy consumption values. In the second test seriesconsiderably lower energy consumption values were obtained especiallyfor fibre finenesses of less than 2.5 μm.

The graph (FIG. 6) shows a comparison between the two test seriesoperated with a transonic rate of gas flow and the conventionalmelt-blowing process operated with the same number of melt orifices 15per cm of die width. All of the average fibre diameters were measured bythe same aerodynamic measuring method. The advantages of a transonicrate of gas flow, in particular for average fibre diameters of less than3 μm are clearly evident.

As far as the physical properties of the fibre web products areconcerned those produced by the process according to the invention aredistinguished by a very low density and very soft handle. No adhesion ofthe fibres could be detected. Irrespective of the distance between thedie and the fibre-collecting belt 9 virtually no flying fibres weredetected even with very fine fibre diameters of <2 μm.

There has thus been shown and described a novel method and device formanufacturing ultrafine fibres from thermoplastic polymers whichfulfills all the objects and advantages sought therefor. Many changes,modifications, variations and other uses and applications of the subjectinvention will, however, become apparent to those skilled in the artafter considering this specification and the accompanying drawings whichdisclose the preferred embodiments thereof. All such changes,modifications, variations and other uses and applications which do notdepart from the spirit and scope of the invention are deemed to becovered by the invention, which is to be limited only by the claimswhich follow.

What is claimed is:
 1. A process for the production of microfibres and non-woven microfibre webs from thermoplastic polymers by the melt-blowing technique, in which the average diameter of the fibres are 0.2 μm to 15 μm, and in which a polymer melt is passed through at least one orifice in a melt-blowing die and is separated into fibres by a gas which impinges on the melt from both sides immediately after its exit from the orifice, said process comprising the steps of:a) conveying the gas through Laval nozzles arranged mirror-symmetrically in relation to the melt orifice and accelerating the gas in the Laval nozzles to supersonic speed; b) passing the gas through flow channels which are arranged downstream of the Laval nozzles and decelerating the gas flow from sonic speed to just below sonic speed in the flow channels, the flow channels having a convergent or substantially constant cross-section in the direction flow; and c) directing the polymer melt stream issuing from the orifice into the gas stream which is discharged from the flow channels.
 2. The process according to claim 1, wherein the mean fibre diameters are 0.5 μm to 10 μm.
 3. A device for the production of microfibres and non-woven microfibre webs from thermoplastic polymers by the melt-blowing technique, in which the mean fibre diameters are 0.2 μm to 15 μm and in which a polymer melt flows through at least one orifice in a melt-blowing die and is separated into fibres by a gas which impinges on the melt from both sides immediately after its exit from the orifices, said device comprising a melt-blowing nozzle with gas supply elements on both sides and gas nozzles directed towards the melt orifice, the improvement wherein the gas nozzles are designed in the form of Laval nozzles with flow channels arranged downstream thereof which have a convergent or substantially constant cross-section, and which are arranged in direct proximity to the wedge-shaped die tip and terminate with a sharp edge at most 3 mm above or below the level of the melt orifice.
 4. A device according to claim 3, wherein the Laval nozzles (30, 31) have a slot-shaped cross-section.
 5. A device according to claim 3, wherein the Laval nozzles (25, 26) each consist of a Laval opening (25) of an orifice diameter of 0.3 to 2 mm.
 6. A device according to claim 4, wherein widened sections (26,32) which lead into the flow channel (27) are arranged downstream of the Laval nozzles (25, 26 or 31,32) and in that the inlet cross-section of the flow channel (27) is 1.0 to 2.5 times the sum of the widened cross-sections of the Laval nozzles, the outlet cross-section is 0.3 to 2.5 times this sum and the length of the flow channel (27) is 1 to 30 times the widened cross-section.
 7. A device according to claim 3, wherein a gas smoothing chamber (23) is arranged upstream of several linearly arranged Laval orifices (25, 26) or of one slot-shaped Laval nozzle (30, 31) and the Laval nozzles (25, 26 or 30, 31) together with corresponding gas smoothing chamber (23) are integrated in the form of individual units to form a modular gas supply element (33a to 33d).
 8. A device according to claim 7, wherein the gas supply elements (33a. . . 33d) the width of which is 25 mm to 500 mm, are arranged in a gas-tight manner next to each other and to the melt-blowing nozzle (18).
 9. A device according to claim 8, wherein the gas supply elements (33a . . . 33d) are displaceable parallel to the wedge-shaped contour of the melt die tip (21), to allow the adjustment of the distance between the Laval nozzles (25, 26 or 31, 32) and the melt orifices (15).
 10. A device according to claim 8, wherein the gas supply elements (33a. . . 33d) have a width of 50 mm to 200 mm.
 11. A device according to claim 3, wherein the mean fiber diameters are 0.5 μm to 10 μm. 